China kept their unprofitable rural slow passenger routes into the distant corners of their country, and now they are converting them to high speed routes.
The Beijing-Guangzhou high-speed line is 2,298km long, and used to be 22 hours but now runs in 8[1]
Being able to go from Penn Station in Manhattan to Union Station in Chicago in 8 hours in a comfortable train would be a serious alternative to air travel, and once the track is upgraded, it would probably be cheaper than existing service. And this is completely doable with decades old technology. Other countries have done comparable things with less.
I don't think we should abandon "legacy" routes just because they are not profitable right now. And even if they are never profitable that alone is not a good enough reason to abandon infrastructure.